1. Field of the Invention
This invention relates generally to sound image placement and, more particularly, to a method and apparatus for producing a specific sound image placement that is independent of the location of the listener relative to the center axis of the loudspeakers.
2. Description of the Background
There have been numerous systems proposed for improving stereo imaging and for creating the impression in the listener that reproduced audio sounds are emanating from various locations within the listening space. Some systems provide reverberation and time delay effects in order to give the listener the feeling that the sounds are being produced within a large concert hall, for example. One system that has been proposed provides for a sound image to be located at points outside of the actual locations of the two transducers playing back the audio signals; such system is disclosed in U.S. patent Ser. No. 239,981 filed Sept. 2, 1988 and assigned to the assignee hereof. This system teaches that a monaural signal may be divided into two signals and those two signals processed in such a fashion that a predetermined phase shift and amplitude alteration differential on a frequency dependent basis exists between these two signals. Upon proper application of this technology, a phantom sound image can be achieved that appears to the listener to be independent of the actual location of the two transducers.
While that system is generally successful in achieving the phantom sound imaging, the listener must generally be somewhere along a center line extending from the two speakers. There is some latitude in this position requirement, of course, yet that latitude does not extend across the entire area in front of the speakers.
Therefore, it has been desired to produce a sound imaging system that is not localized in its effects and in which a single listener or multiple listeners can be ranged across the front of the two speakers at several locations yet still all perceive a similar phantom sound image location.